1. Field of the Invention
The present invention relates to switch assemblies and, more particularly, relates to a method and apparatus for monitoring a contact in a switch assembly.
2. Discussion of the Related Art
Electrical switches, such as pushbuttons or rotary switches used for the control of industrial equipment, are typically mounted onto a front panel of a cabinet so that the manipulated portion of the switch (termed the "operator") projects out from and is accessible at the front of the cabinet.
For a pushbutton switch, a hole may be punched in the cabinet of sufficient diameter to accommodate the pushbutton and a surrounding threaded shaft. The shaft and pushbutton are inserted through the hole, and a threaded retaining nut is placed over the shaft and tightened to securely affix the switch to the panel. The panel is thus sandwiched between the switch body and the retaining nut.
The end of the switch operator protruding inside of the panel may be snapped or otherwise mounted onto one side of a latch assembly, and a contact block or a plurality of contact blocks are mounted onto the other side of the latch assembly. The contact blocks are electrically connected to the circuit or circuits that the switch is to control.
Contact blocks typically comprise housings that contain normally open and/or normally closed contacts. A normally open contact may be used, for example, when a user wishes to activate a specified function by actuating the operator, thereby closing the normally open contact. When the operator switch is deactivated, a plunger returns to its normal position, thereby opening the normally open contact and terminating the controlled function.
A normally closed contact may be used when a user wishes to stop an ongoing function. One common example of a normally closed contact is an Emergency Stop (EStop) function which is activated when the user wishes to immediately terminate the controlled function due, e.g. to a malfunction in the process or the development of a situation that may cause damage to the product line or the operating equipment. In this situation, when the switch operator is actuated, the normally closed contact opens and remains open until the operator is returned to its normal state, thereby closing the normally closed contact and resuming the controlled function.
In such systems, the user assumes a risk that the normally closed contact may become mechanically disengaged from the switch operator. Such a situation may occur, for example, if the latch assembly is damaged or not properly mounted onto the switch operator and therefore becomes detached during operation. Alternatively, the contact block may be damaged or improperly mounted. Even though, in these situations, the contact block is mechanically disconnected from the switch operator, the normally closed contact remains closed, thereby permitting the continuous operation of the controlled function. As a result, when the normally closed contact is functioning as an E-Stop, for example, the controlled function will remain in operation even though the contact block is no longer mechanically engaged with the switch operator.
Currently, one known way to ensure an operable state of a normally closed switch is to test it by intermittently activating the switch operator. If, after activation, the controlled function is nonresponsive, then the user will become aware of a problem in the switch assembly and may take corrective measures. However, this method of detection is quite inefficient and results in considerable unnecessary down-time, thereby increasing cost. Furthermore, this method is unreliable as situations may arise that require the activation of an E-Stop that has become non-operational since the last test.
The need has therefore arisen to implement a method and apparatus for detecting when the normally closed contact becomes mechanically disengaged from the switch operator in an efficient and reliable manner.